Turbulence

Occasionally, a Plane will experience turbulence during a flight, where it will, for some reason, be flipped upside down for a few seconds, a siren will be heard, and all of the passengers on-board will start screaming, as will the pilot. Such occurrences usually mean that the plane is about to land in a city currently experiencing (and possibly closed by) bad weather, though not always. Conversely, turbulence may also happen to the plane for no reason, even there isn't any bad weather in sight.